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Full text of "The Cambridge Natural History"

THE BRAIN                                       75

and is a piece of evidence in favour of the high position of the
mammals.
The oviducal apparatus of the mammal is more specialised
than that of lower vertebrates. It is most simple, as might be
imagined, in the egg-laying Monotremes, where, indeed, it is on
the same level as that of reptiles. But in the Eutheria the
fmibriated mouth of the oviduct passes into a narrow and wind-
ing tube, the Fallopian tube; this widens into a uterus, and the
two uteri combine into a single tube in the higher forms. They
are called the Monodelphia on this account. In the Marsupials
the uteri are distinct though they often join above, and from
this junction depends a median " uterus." After the uterus or
the uteri follows in every case a single vagina.
The testes of the Mammalia, like those of other vertebrates,
occupy primitively a position within the body cavity precisely
corresponding to that of the ovaries. And in the lowly-organised
Monotremata, and some other forms., such as the "Whales, they
retain that primitive position within the body. It is, however,
distinctive of the Mammalia as opposed to lower vertebrates that
the testes descend later into a scrotum, which is simply a pro-
trusion of the skin of the body surrounded by muscles, and, of
course, containing a section of the body cavity in which lie the
testes. The penis of the Mammalia, represented by the clitoris
and associated structures in the female, is of a structure entirely
peculiar to this group.
The Brain.-Inasmuch as Professor Wiedersheim has said
with perfect truth that e( the brain of the extinct Ungulate
Dinocer&s shows so striking a likeness to that of a lizard that
one would be compelled to explain it as that of a lizard without
a knowledge of the skeleton," it is clear that to define thp
mammalian brain is a difficult matter. The existing Mammalia,
however, all possess brains which can be readily distinguished
from those of vertebrates lying lower in the scale. They are
of relatively large size, brought about mainly by the dimensions
of the cerebral hemispheres, which have an importance in this
class of vertebrates that they have not elsewhere. Coupled
with this large size of the hemispheres is a more elaborate
system of transverse commissures uniting the two; and this
culminates in the higher Mammalia, where the corpus callosum
attains a large size and great physiological importance. A